Sequential extraction schemes provide information on the mobility of heavy metals from polluted sediments. The lack of uniformity of these schemes, however, does not allow results to be compared nor the procedures to be validated. In 1987 the Community Bureau of Reference, (BCR, now Standard, Measurements and Testing Programme) of the European Commission launched a programme to harmonise single and sequential extraction schemes for soils and sediments. As a result certified reference material CRM 601 (lake sediment) was prepared and certified for the extractable contents of some trace elements, following a standardised sequential (three-step) extraction procedure. In order to verify the long-term stability of the material an intercomparison exercise was designed with the participation of eight expert laboratories. The results confirm the stability of the extractable contents of Cd, Cr, Ni, Pb and Zn in this type of material, and demonstrate that the preparation of this kind of CRM can be achieved.